Automation System
A disadvantage of Numerical Control (NC) is:

the computer is not reliable
the tape and tape reader are not reliable
the machine tool can easily overheat
one operator is needed for each machine.
None of these

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Automation System
Time proportioning control:

a and b above.
None of these
cannot use a TM990 micro¬computer
is less efficient than normal PID control
can be used on a PM550 programmable process controller

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